Abstract: Through field exposure tests of concrete in marine tidal environment, the influence of fly ash additive on chloride diffusivity and its time dependency is studied. First, free chloride concentration of the concrete is measured at four exposure times for fly ash contents of 0%, 20%, 30%, 40% and 50%, and porosity of the concrete is tested using a mercury intrusion porosimetry (MIP). Second, apparent diffusion coefficient of the concrete is calculated by Fick’s second law and used to determine instantaneous diffusion coefficients. An equation is formulated to describe the relationship of fly ash content versus diffusion coefficient. Finally, a time-dependent model of instantaneous diffusion coefficients is developed and verified against the measurement of concentrations. Calculation results of this model show that both apparent and instantaneous diffusion coefficients decrease with exposure time and fly ash content. And the time dependency of diffusion coefficients indicates that chloride resistance of fly ash concrete is greater in the early stage than that in the late stage. The age reduction factor of diffusion coefficients keeps increasing with fly ash content. However, at the exposure time of 360 d, this factor of instantaneous diffusion tends to be stable. Thus, our time-varying model of instantaneous chloride diffusion coefficients after 360 d exposure is accurate in prediction of free chloride concentration of concrete.

Key words: tidal environment, fly ash concrete, apparent chloride diffusion coefficient, instantaneous chloride diffusion coefficient, time dependency